Interpretive Summary: A large amount of carbon dioxide is retained in trees, which helps to offset the rise in atmospheric levels of carbon dioxide attributable to the burning of fossil fuels. Considerable amounts of carbon dioxide are also stored in forest soils, but the amounts sequestered and the mechanisms involved are not well understood. Therefore, we quantified the amounts, distribution, and chemical composition of organic matter in a temperate forest soil in Northern Appalachia. The forest soil had relatively high levels of soil carbon near the surface due to high surface litter inputs, but this was more readily decomposable than the carbon found deeper in the soil profile. Most of the chemically stabilized carbon was found in the sub-soil horizons, thus they probably are more important for long-term storage of carbon than the litter layer. Knowing this it may be possible to enhance storage of carbon in these horizons by planting deep-rooted tree species that release large amounts of organic matter into the subsoil as a method to migrated human-induced increases in atmospheric carbon dioxide levels.

Technical Abstract:
The soil organic carbon (SOC) pool stored in sub-soil horizons in forests plays an important role in the global C cycle. Strategies are needed to increase the sub-soil SOC pool in forests because the turnover time of SOC increases with increase in soil depth as sub-soil SOC is chemically and physically stabilized. We compared the total SOC and total nitrogen (TN) pools, chemically and physically separated SOC fractions, and C and N pools in fine roots in a soil pedon in an oak-hickory forest type consisting of white oak (Quercus alba L.) and red oak (Quercus rubra L.), with yellow-poplar (Liriodendron tulipifera L.) at the North Appalachian Experimental Watershed near Coshocton, Ohio, USA. The SOC pool (Mg/ha) was the highest in the A horizon (47.4), and smaller in the Bt1 (6.9) and Bt2 (6.7) horizons. The SOC and TN concentrations sharply decreased with depth. Fine root C and N pools (Mg/ha) were much larger in the A horizon (0.71 and 0.025) than in underlying horizons. Although only 22% of the SOC pool was stored below the A horizon, 58% of the chemically stabilized and 31% of the physically stabilized SOC fractions pool occurred in the sub-soil horizons. Thus, studies are needed to test if forest management can strengthen the stabilized SOC pool in sub-soil horizons to mitigate the human-induced climate change.